1) Field of the Invention
This invention pertains to the field of digital communication systems, and more particularly to digital communication signal detection and synchronization.
2) Background of the Related Art
Many modern digital communications systems utilize burst type transmissions in which a relatively small number of information bits are transmitted by sending a sequence of a small number of waveforms, say xe2x80x9cNxe2x80x9d in number, termed xe2x80x9csymbols,xe2x80x9d each of which symbols can assume one of xe2x80x9cQxe2x80x9d possible shapes. As an example, Q=4 for the case of quadraphase shift keying (QPSK).
In a burst-type communication system it is important to rapidly detect the onset of the signal at the receiver and to perform time synchronization to it. To facilitate this, in many burst-type communication systems a burst consists of a initial set of symbols, termed a xe2x80x9cpreamble,xe2x80x9d which permits rapid synchronization, and a subsequent set of symbols, termed a xe2x80x9cpayload,xe2x80x9d which represents the actual message data. Some systems also include special data in the middle of the burst, or at the end, which is termed a xe2x80x9cpostamble.xe2x80x9d
Time synchronization for a burst-type communication system includes two aspects: (1) determining the timing of the symbols, and (2) determining the onset of the actual payload data carried by the burst. It is important that such synchronization be performed using the preamble data so that the payload data may be correctly decoded. Determining the onset of payload data in a burst transmission is typically performed through the transmission during the preamble of a specific sequence of symbols, termed a xe2x80x9cunique word.xe2x80x9d
Time synchronization is to be distinguished from determination of received signal carrier frequency and phase, which is termed xe2x80x9ccarrier synchronization.xe2x80x9d
In addition to allowing time synchronization, the preamble may be used to transmit a limited amount of information that describes the subsequent payload data. For example, such information may specify the length of the payload (number of symbols), the modulation format of the payload data, or other information.
In conventional systems the above information is sent in the preamble by the normal manner of associating individual symbols with information bits. However, in situations in which the received signal level is low, such individual symbols may be obscured by noise such that this conventional approach will lead to very high error rates.
Accordingly, it would be advantageous to provide a system and method which will perform both time synchronization and information transmission via a preamble when the input signal level is low. Other and further objects and advantages will appear hereinafter.
The present invention comprises a combined synchronization and information transfer method and system in a burst-type communication system.
In one burst-type communication system of the present invention having a transmitter and a receiver, a method of communicating information in a preamble synchronization sequence of a burst transmission includes selecting, at the transmitter, a selected one of a plurality of initial detection sequences, representing one or more first preamble information bits; selecting, at the transmitter, a selected one of a plurality of unique words, representing one or more second preamble information bits; transmitting, at the transmitter, said preamble synchronization sequence comprising said selected one initial detection sequence and said selected one unique word; receiving, at the receiver, said preamble synchronization sequence; detecting, at the receiver, said one initial detection sequence to produce first time synchronization information and to receive said one or more first preamble information bits; and detecting, at the receiver, said one unique word to produce second time synchronization information and to receive said one or more second preamble information bits.
In another burst-type communication system of the present invention, a receiver includes a differential detector receiving a differentially encoded synchronization sequence and providing a base synchronization sequence including a first plurality of symbols, comprising an initial detection sequence, and a second plurality of symbols, comprising a unique word; an integrator receiving the base synchronization sequence and integrating the first plurality of symbols to produce an initial detection signal; a threshold detector receiving the first plurality of symbols, detecting the initial detection, producing an initial detection strobe comprising symbol timing information, and providing a first preamble information bit; and a unique word detector receiving the base synchronization sequence, detecting the unique word, and producing a second preamble information bit.
In another burst-type communication system of the present invention, a transmitter transmits a preamble synchronization sequence, the preamble synchronization sequence including an encoded initial detection sequence comprising a plurality of symbols representing one or more first preamble information bits; and an encoded unique word comprising a plurality of symbols representing one or more second preamble information bits.
In one aspect of the invention, a burst-type communication system employs differential encoding and decoding. Differential decoding operates on the differentially encoded data to remove the effects of signal carrier frequency uncertainty from the signal and enables processing a large number of symbols by a subsequent matched filter without the deteriorating effects of such carrier frequency uncertainty.
In another aspect of the invention, a burst-type communication system employs matched filtering, permitting integration of a received signal over a large number of symbols, thereby producing a strong detection output even when the individual signal symbols are weak relative to the noise level. This integration may permit detection even when, for example, the power of the symbols equals the power of the noise.
In yet another aspect of the present invention, each burst transmission in a burst-type communication system includes one of a number xe2x80x9cSxe2x80x9d allowed synchronization sequences, S greater than 1. A receiver includes one or more multiple matched filters to decide which of the S allowed synchronization sequences was transmitted.
In a further aspect of the present invention, one of two synchronization sequences may be transmitted, such that after differential decoding, the two synchronization sequences only differ from one another by a polarity inversion. In this case a single matched filter may be used to detect both synchronization sequences.
In yet a further aspect of the present invention, one or more bits of preamble information is communicated in a synchronization sequence in a burst transmission. Each burst may include one of xe2x80x9cSxe2x80x9d allowed synchronization sequences. The selection of a particular synchronization sequence among the xe2x80x9cSxe2x80x9d allowed synchronization sequences communicates log2 (S) preamble information bits. Each synchronization sequence may be divided into two or more sections with one or more bits of information communicated in each section.